program C3;

{$APPTYPE CONSOLE}

uses
  SysUtils;

var
  mass : array of integer;
  sum, n, i : integer;

begin
  sum:=0 ;
  readln(n);
  setlength(mass, n);
  for i:=0 to n-1 do
  mass[i]:= i;
  if (n mod 2 <> 0) then
  begin
  writeln ('error');
  end;
  if (n mod 2 = 0) then
  for i := 0 to n do
  begin
  sum := sum + mass[i];
  end;
  writeln(sum);
  readln;

end.
 